Fragment molecular orbital-based molecular dynamics (FMO-MD) method with MP2 gradient

The energy gradient of the second-order Møller–Plesset perturbation theory (MP2) has been implemented in conjunction with the fragment molecular orbital-based molecular dynamics (FMO-MD) method including up to three-body correction (FMO3). A hybrid integral-direct approach of both atomic and molecular orbital indices was utilized with parallelism for the gradient calculations. A droplet model consisting of 64 water molecules was then simulated with the 6-31G∗ basis set. The first peak position of O–O radial distribution function was evaluated to be 2.75 Å at the FMO3-MP2 level, whereas the corresponding Hartree–Fock (FMO3-HF) value was 2.89 Å. Comparison with an X-ray value of 2.73 Å indicated better reliability of the MP2 gradient for FMO-MD.